A CDMA-based random access channel (RACH) provides a common uplink packet transport from a mobile station (MS) to a base station (BS). The RACH is typically composed of many sub-channels defined by preamble code sequences over a well-defined timeslot (RACH sub-channels). Typically, there are a number of base stations and a plurality of mobile stations. Each MS has a transmitter and receiver. An uplink (UL) is from the MS to the BS. A downlink (DL) is from the BS to the MS. The BS broadcasts common messages and control signals to a plurality of mobile stations through the DL broadcast and control channel (BCCH), typically embedded in a broadcast, paging and common-control channel (BPCCH). The broadcast message on the BCCH channel contains information such as the available random-access preamble codes, their associated timeslots (i.e., the RACH sub-channels), ACK messages, etc.
The channel resource allocation of RACH is contention based. A simplified example of the signals exchanged between an MS and a BS for a RACH service follows. An MS listens to the message on the BCCH channel and transmits one or more random access signals over an uplink physical common channel, in access slots defined in relation to a frame-timing signal derived from receipt of the common synchronization channel. The random access signal contains a preamble code corresponding to a RACH sub-channel. When the BS receives a random access signal at an adequately detectable power level, it transmits back an acknowledgement (ACK), containing a code that corresponds to the access preamble code.
If the MS does not receive an acknowledgement with a set time; it retransmits its access attempt signal, at an increased power level. The MS ceases transmission of the random access signals when it receives the corresponding ACK signal from the base station. If the MS successfully receives the acknowledgement corresponding to the access preamble code that it transmitted, the MS proceeds to the next phase in the transmission process, referred to generally as transmission of data over a dedicated timeslot in an uplink data channel.
Alternatively, the MS will cease transmission of random access signals if it has transmitted the maximum allowed number of random access signals (i.e. time-out) or if it has received a negative acknowledge (NACK) from the BS. The MS assumes that its access attempt has failed, so it backs off and waits for some period of time before initiating another access attempt.
Since multiple mobile stations may be accessing the BS at the same time, they may be simultaneously generating increasingly powerful and interfering transmissions, which is undesirable. Various methods of power control were developed to reduce the excessive signal power.
Published International application WO00/03499 (Kim et al.) teaches a transmission preamble power control methodology to slowly adjust the transmission power of the access preamble, based on the combination of a closed-loop power control bit and an open-loop power control bit. In the disclosed access method, the mobile station periodically transmits a preamble signal, and each transmission uses an increased power. Upon receipt of an acknowledgement from the base station, the mobile station accesses the reverse common channel. During the access procedure, the mobile station measures the strength of a signal received from the base station and generates an open loop power control signal. The mobile station also receives a power control bit in each of a series of transmissions over the forward channel. The mobile station accumulates the received signal strength measurement and the received power control bits over time, and it uses those two accumulated signals to control the transmission power of the preamble signal. With this approach, the open-loop power control bit only conveys two kinds of information on the power control, power-up or power-down. The closed-loop power control bit only commands power-down on the transmitted preamble signal. It does not command power-up. Therefore the resulting transmission preamble power control signal does not reveal how much the power should be adjusted on the transmitted preamble signal, for any particular instant or transmission cycle. Also, this control technique requires time to accumulate the necessary control information. While the aforementioned power control method may be sufficient in a long preamble ramp-up process, it is not adequate for a fast packet-access communications system to ensure high-throughput.
Also, in a high-capacity system, the random-access resource is very limited. For example, in a given system, there are up to 64 RACH sub-channels. But the actual number of RACH sub-channels in a basic frame that can be assigned by the BS depends on the load of the network. When there is a light load, up to or more than 64 RACH sub-channels can be assigned to mobile stations for reducing the wait-time of gaining access to the network. But when the network is heavily loaded, a minimum of up to 32 RACH sub-channels is assigned by the BS to reduce the load. For a given number of mobile stations attempting to access the network, the reduced number of sub-channels means that on average each individual mobile station will encounter a longer wait time before successfully accessing the network.
Hence, there is a need for a technique to achieve fast channel-access, without imposing access delays during initial power control.